Low temperature growth of highly transparent nanocrystalline diamond films on quartz glass by hot filament chemical vapor deposition

Highly transparent and hard nanocrystalline diamond (NCD) films were prepared on quartz glass by hot filament chemical vapor deposition (HFCVD). The effects of total gas pressure, substrate's temperature, and concentration of CH4 on the grain size, surface's roughness and hardness, growth rate, as well as the optical properties of NCD films were investigated. The results indicated that with a low total gas pressure and high CH4 concentration, high frequency of secondary nucleation can be obtained. In addition, low substrate temperature can increase the rate of the hydrogen atom etched sp2 graphite carbon in the film, yielding a smooth surface of NCD films and very high sp3 content. Under optimized conditions, the hardness can be enhanced up to 65 Gpa, with 80% maximum transmittance in the visible light region. The aforementioned reaction platform outcomes a 1.2 μm thickness of NCD coating with a low root-mean-square (r.m.s.) surface roughness around 12–13 nm and a high growth rate around 1 μm/h. The influences of the total gas pressure, substrate's temperature, and CH4 concentration for growing NCD films were also discussed in this paper.